U.S. patent number 6,630,797 [Application Number 09/883,445] was granted by the patent office on 2003-10-07 for high efficiency driver apparatus for driving a cold cathode fluorescent lamp.
This patent grant is currently assigned to Koninklijke Philips Electronics N.V.. Invention is credited to Jinrong Qian, DaFeng Weng.
United States Patent |
6,630,797 |
Qian , et al. |
October 7, 2003 |
High efficiency driver apparatus for driving a cold cathode
fluorescent lamp
Abstract
An inverter circuit for a gas discharge lamp having a primary
circuit having a DC voltage supply, a transformer, a switching
circuit including a first switch and a second switch for
controlling a conduction state of the inverter circuit; a tank
circuit having a resonant inductor and a resonant capacitor, the
lamp load being coupled with the resonant capacitor; and a
capacitor coupled to the first and second switches for maintaining
a voltage across a primary winding of said transformer.
Accordingly, the required turns ratio of the transformer is reduced
by half which reduces the power loss in the transformer, thereby
improving circuit efficiency. In addition, energy stored in a
leakage inductance, which is otherwise dissipated across the
switches of the push-pull switch configuration in the prior art, is
recovered or captured by the clamping capacitor, thereby preventing
the occurrence of voltage spikes across the switches.
Inventors: |
Qian; Jinrong
(Croton-on-Hudson, NY), Weng; DaFeng (San Jose, CA) |
Assignee: |
Koninklijke Philips Electronics
N.V. (Eindhoven, NL)
|
Family
ID: |
25382592 |
Appl.
No.: |
09/883,445 |
Filed: |
June 18, 2001 |
Current U.S.
Class: |
315/224; 315/244;
315/DIG.7 |
Current CPC
Class: |
H05B
41/2821 (20130101); Y10S 315/07 (20130101) |
Current International
Class: |
H05B
41/282 (20060101); H05B 41/28 (20060101); H05B
041/29 () |
Field of
Search: |
;315/29R,219,224,225,226,244,291,307,DIG.7,239,241R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Nguyen; Hoang
Assistant Examiner: Tran; Thuy Vinh
Claims
We claim:
1. An inverter circuit for driving a gas discharge lamp load in a
load circuit, said inverter circuit comprising: a primary circuit
including a DC voltage supply, a transformer including a primary
winding and a secondary winding for coupling said primary circuit
to the load circuit, and a switching circuit having a first switch
and second switch for controlling a conduction state of said
inverter circuit, wherein said primary winding is connected to a
midpoint connection terminal of said switching circuit; a tank
circuit including a resonant inductor within said primary circuit,
and a resonant capacitor coupled to the lamp load; and a capacitor
coupled to said first switch and said second switch for maintaining
a voltage across said primary winding.
2. The inverter circuit of claim 1, wherein said resonant inductor
is coupled in series with said primary winding of said
transformer.
3. The inverter circuit of claim 1, wherein said primary circuit
further includes said capacitor.
4. An inverter circuit for driving a gas discharge lamp load in a
load circuit, said inverter circuit comprising: a primary circuit
including a DC voltage supply, a transformer including a primary
winding and a secondary winding for coupling said primary circuit
to the load circuit, and a switching circuit having a first switch
and second switch for controlling a conduction state of said
inverter circuit, wherein said primary winding is connected to a
midpoint connection terminal of said switching circuit; a tank
circuit including a resonant capacitor and a resonant inductor,
wherein the lamp load is coupled in series with said resonant
capacitor and said resonant inductor; and a capacitor coupled to
said first switch and said second switch for maintaining a voltage
across said primary winding.
5. The inverter circuit of claim 4, wherein said primary circuit
further includes said capacitor.
6. An inverter circuit for driving a gas discharge lamp load in a
load circuit, said an inverter circuit comprising: a primary
circuit including a DC voltage supply, a transformer including a
primary winding and a secondary winding for coupling said primary
circuit to the load circuit, and a switching circuit having a first
switch and second switch for controlling a conduction state of said
inverter circuit, wherein said primary winding is connected to a
midpoint connection terminal of said switching circuit; a tank
circuit including a resonant inductor and a resonant capacitor, the
lamp load being coupled to said resonant capacitor; and a capacitor
coupled to said first switch and said second switch for maintaining
a voltage across said primary winding, wherein said resonant
inductor provides a boost function to said capacitor.
7. The inverter circuit of claim 6, wherein said primary circuit
further includes said resonant inductor.
8. The inverter circuit of claim 7, wherein said resonant inductor
is coupled in series with said primary winding of said
transformer.
9. The inverter circuit of claim 6, wherein said primary circuit
further includes said capacitor.
10. A method of eliminating voltage spikes in an inverter circuit
for a gas discharge lamp, said method comprising: providing a
primary circuit including a DC voltage supply, a switching circuit
having a first switch and a second switch for controlling a
conduction state of the inverter circuit, and a transformer having
a primary winding connected to a midpoint connection terminal of
said switching circuit; providing a tank circuit having a resonant
inductor and a resonant capacitor, the lamp load being coupled with
the resonant capacitor; and providing a capacitor coupled to the
first switch and the second switch for maintaining a voltage across
the primary winding; and providing a boost function by the resonant
inductor to the capacitor.
11. A method of eliminating voltage spikes in an inverter circuit
for a gas discharge lamp, said method comprising: providing a
primary circuit including a DC voltage supply, a switching circuit
having a first switch and a second switch for controlling a
conduction state of the inverter circuit, and a transformer having
a primary winding connected to a midpoint connection terminal of
said switching circuit; providing a tank circuit having a resonant
inductor and a resonant capacitor, the lamp load being coupled with
the resonant capacitor; and providing a capacitor coupled to the
first switch and the second switch for maintaining a voltage across
the primary winding; and recovering leakage energy from the
transformer in each of a plurality of switching cycles of the
inverter circuit.
12. An inverter circuit for driving a gas discharge lamp load in a
load circuit, said inverter circuit comprising: a primary circuit
including a DC voltage supply, a transformer including a primary
winding and a secondary winding for coupling said primacy circuit
to the load circuit, and a switching circuit having a first switch
and second switch for controlling a conduction state of said
inverter circuit; a tank circuit including a resonant inductor
within said primary circuit, and a resonant capacitor coupled to
the lamp load; and a capacitor for maintaining a voltage across
said primary winding, wherein said primary winding is connected to
said first switch and said capacitor, and wherein said capacitor is
coupled in series with said second switch.
13. The inverter circuit according to claim 12, wherein said
resonant inductor is coupled in series with said primary
winding.
14. The inverter circuit of claim 12, wherein said primary circuit
further includes said capacitor.
15. An inverter circuit for driving a gas discharge lamp load in a
load circuit, said inverter circuit comprising: a primary circuit
including a DC voltage supply, a transformer including a primary
winding and a secondary winding for coupling said primary circuit
to the load circuit, and a switching circuit having a first switch
and second switch for controlling a conduction state of said
inverter circuit; a tank circuit including a resonant inductor and
a resonant capacitor, the lamp load being coupled to sad resonant
capacitor; and a capacitor for maintaining a voltage across said
primary winding, wherein said primary winding is connected to said
first switch and said capacitor, and wherein said capacitor is
coupled in series with said second switch, and wherein said
resonant inductor provides a boost function to said capacitor.
16. The inverter circuit according to claim 15, wherein said
primary circuit includes said resonant inductor.
17. The inverter circuit according to claim 16, wherein said
resonant inductor is coupled in series with said primary
winding.
18. The inverter circuit of claim 17, wherein said primary circuit
further includes said capacitor.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device for driving a cold
cathode fluorescent lamp (CCFL) used as a backlight of a liquid
crystal display.
2. Description of the Related Art
Similar to a conventional hot-cathode fluorescent lamp ("FL") used
for office and home lighting, CCFLs are high-efficiency, long-life
light sources. By comparison, incandescent lamps have efficiency in
the range of 15 to 25 lumens per watt, while both FLs and CCFLs
have efficiency in the range of 40 to 60 lumens per watt.
Furthermore, the average life of an incandescent lamp is only about
1,000 hours. However, FLs and CCFLs, on average, last for 10,000
hours or more.
The main difference between a hot-cathode FL and a CCFL is that the
CCFL omits filaments that are included in a FL. Due to their
simpler mechanical construction and high efficiency, miniature
CCFLs are generally used as a source of back lighting for Liquid
Crystal Displays ("LCDs"). LCDs, whether color or monochrome, are
widely used as displays in portable computers and televisions, and
in instrument panels of airplanes and automobiles.
However, starting and operating a CCFL requires a high alternating
current ("ac") voltage. Typical starting voltage is around 1,000
volts AC ("Vac"), and typical operating voltage is about 600 Vac.
To generate such a high ac voltage from a dc power source such as a
rechargeable battery, portable computers and televisions, and
instrument panels, include a dc-to-ac inverter having a step-up
transformer.
In the push-pull configuration illustrated in FIG. 1, L.sub.k1 and
Lk2 are the leakage inductances of the transformer T, D.sub.S1 and
C.sub.s1 are the body diode and internal capacitance of switch S1,
respectively, and D.sub.s2 and C.sub.s2 are the respective body
diode and internal capacitance of switch S2. Winding N3 is coupled
with windings N1 and N2. Inductor Lr, is a resonant inductor
including a leakage inductance of transformer T. Inductor Lr and
capacitor Cr form a resonant tank to provide a high frequency
voltage to the load, R.sub.o.
FIGS. 2a-2d illustrate typical switching waveforms associated with
the circuit of FIG. 1. Referring first to FIG. 2a, at the point in
time when switch S1 is turned off (t0) energy stored in the leakage
inductance L.sub.k1 is released to charge the capacitance Cs1 which
causes an undesirable voltage spike across switch S1, as
illustrated in FIG. 2c. Another problem associated with the circuit
configuration of FIG. 1 is that the high voltage spike requires
that switches S1 and S2 have high voltage breakdown voltage
ratings.
At time t1, the gate signal (See FIG. 2b) of switch S2 is applied
allowing switch S2 to be turned on at zero voltage (not shown). S2
carries the primary winding current.
As shown in FIG. 2d, a second voltage spike occurs at time t2 at
switch S2, the point at which switch S2 is turned off. This voltage
spike is the result of the release of energy from the leakage
inductance L.sub.k2.
Referring now to FIG. 3, one prior art solution for eliminating or
minimizing the undesirable voltage spikes is through the use of
passive snubber circuits (R-C-D) for switch S1 and (R-C-D) for
switch S2, respectively. The passive snubber circuits are designed
to absorb the leakage energy of the transformer (L.sub.k1,
L.sub.k2). An undesirable consequence of using snubber circuits is
that the converter circuit has a lower conversion efficiency by
virtue of having to dissipate the undesirable leakage energies.
Another type of conventional ballast, illustrated in FIG. 4,
employs a half-bridge inverter circuit configuration. The
half-bridge switching circuit includes switches S1 and S2, resonant
inductor L.sub.r and resonant capacitor C.sub.r. Inductor L.sub.r
could represent the leakage inductance or a separate inductance in
the case where the leakage inductance is insignificant. C.sub.r
could represent a combination of the winding capacitance and shield
capacitance of the lamp. C.sub.d represents a DC blocking
capacitor. The input voltage, V.sub.in, is typically around 12 V.
Until the CCFL or load (R.sub.L) is "struck" or ignited, the lamp
will not conduct a current with an applied terminal voltage that is
less than the strike voltage, e.g., the terminal voltage can be as
large as 1000 Volts. Once an electrical arc is struck inside the
CCFL, the terminal voltage may fall to a run voltage that is
approximately 1/3 the value of the strike voltage over a relatively
wide range of input currents. To achieve voltages on the order of
1000 volts, a high voltage gain of the resonant inverter is
required in addition to a high turns ratio of the isolation
transformer. However, given that the peak excitation voltage
V.sub.x of the resonant tank is only one-half the input voltage,
the resonant inverter voltage gain is restricted. Therefore, the
only means of achieving a strike voltage on the order of 1000 volts
is to require that the transformer have a very high turns ratio.
This is problematic, however, in that a high turns ratio
transformer is characteristically leaky and therefore not
efficient.
Accordingly, it is desirable to provide an improved ballast which
is more efficient in operation than a conventional ballast whether
of the push-pull or half-bridge type while reducing or
substantially eliminating spike voltages.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide an
inverter circuit which eliminates or substantially reduces voltage
spikes associated with switching elements in a push-pull switch
configuration.
It is a further object of the invention to provide an inverter
circuit which recovers leakage energy associated with an isolation
transformer to improve circuit efficiency.
It is yet a further object of the invention to provide an inverter
circuit which reduces the turns ratio of the isolation transformer
to reduce power losses in the transformer to further improve
circuit efficiency.
In accordance with an embodiment of the present invention, there is
provided an inverter circuit and a method for efficiently
converting a direct current (DC) signal into an alternating current
(AC) signal for driving a load such as a cold cathode fluorescent
lamp. The inverter circuit includes a resonant tank circuit having
a resonant inductor and resonant capacitor and coupled via a
transformer between a DC signal source and a common terminal of a
half-bridge switch configuration. A voltage clamping capacitor is
connected to a second and third terminal of the half-bridge switch
configuration. A voltage difference between the capacitor voltage
and the supply (i.e., input) voltage is applied to the terminals of
the resonant tank. The voltage difference across the resonant tank
is nominally twice the voltage of prior art configurations.
The inverter circuit according to the present invention includes a
primary circuit having a DC voltage supply, a transformer coupling
said primary and load circuits, a switching circuit comprising a
first switch and a second switch for controlling a conduction state
of said inverter circuit; a tank circuit having a resonant inductor
and a resonant capacitor, the lamp load being coupled with the
resonant capacitor; and a capacitor coupled to the first and second
switches for maintaining a voltage across a primary winding of said
transformer.
Accordingly, the required turns ratio of the transformer is reduced
by half, as compared to prior art inverter circuits, thereby
reducing the power loss in the transformer which improves circuit
efficiency.
In accordance with another aspect of the present invention, the
leakage energy stored in a leakage inductance associated with the
transformer is recovered or captured by the clamping capacitor
thereby preventing or substantially reducing the occurrence of
voltage spikes across the switches which comprise the half-bridge
switching configuration. As described above, in one prior art
configuration, this leakage inductance, when released, charges a
capacitance associated with the push-pull switches which causes
voltage spikes across the switches. An additional advantage of
capturing the leakage current is that the voltage ratings of the
switches is significantly reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing features of the present invention will become more
readily apparent and may be understood by referring to the
following detailed description of an illustrative embodiment of the
present invention, taken in conjunction with the accompanying
drawings, where:
FIG. 1 is a circuit diagram illustrating an LCD backlighting
inverter circuit of the prior art;
FIGS. 2a-2d illustrate representative waveforms present in the
circuit of FIG. 1;
FIG. 3 is a circuit diagram illustrating an LCD backlighting
inverter circuit of the prior art;
FIG. 4 is a circuit diagram illustrating an LCD backlighting
inverter circuit of the prior art;
FIG. 5 is a circuit diagram illustrating an LCD backlighting
inverter circuit in accordance with an embodiment of the present
invention;
FIGS. 6a-6d illustrate representative waveforms present in the
circuit of FIG. 5;
FIG. 7 is a circuit diagram illustrating an LCD backlighting
inverter circuit in accordance with an embodiment of the present
invention;
FIG. 8 is a circuit diagram illustrating an LCD backlighting
inverter circuit in accordance with an embodiment of the present
invention; and
FIG. 9 is a circuit diagram illustrating an LCD backlighting
inverter circuit in accordance with an embodiment of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A circuit configuration is provided to obviate voltage spikes which
occur at turn-off for each push-pull switch of an inverter circuit.
Additionally, the circuit configuration is more efficient than
conventional inverter circuit configurations.
Turning now to FIG. 5, an exemplary schematic of the inverter
circuit 10 displays one embodiment of the inventive circuit
configuration connected to a load R.sub.L. Load R.sub.L can be, but
is not limited to a fluorescent lamp of the cold cathode type. The
light from load R.sub.L can be used to illuminate a liquid crystal
display (LCD) of a computer. Load R.sub.L is connected to a
secondary winding of a transformer T. Transformer T includes one
primary winding, N.sub.p, and one secondary winding N.sub.s. A
resonant circuit is formed by a resonant inductor Lr and a resonant
capacitor Cr. Other than resonant inductor Lr and resonant
capacitor Cr, there is no other discrete inductor or capacitor
included which substantially affects the resonant frequency of the
resonant circuit. There is also no discrete ballasting element,
typically a capacitor, in series with load R.sub.L. The elimination
of these discrete components from the resonant circuit or serially
connected to the load R.sub.L reduces the parts count and cost of
the inverter circuit 10.
The half-bridge switching circuit (i.e., switching stage) includes
switches S1 and S2. These switches are turned on and off by a drive
control circuit (not shown). Switches S1 and S2 are never turned on
at the same time and have ON time duty ratios of slightly less than
50% as shown in FIGS. 6A and 6B. A small dead time during which
both switches are turned off is required to permit the zero voltage
switching to be implemented. An output of the primary winding
N.sub.p of the transformer T is connected to a midpoint connection
terminal of the half-bridge switching circuit (See point B in FIG.
5). A clamping capacitor C.sub.o is connected in parallel with the
half-bridge switching circuit. The inverter circuit 10 is sourced
by a 12 V DC power supply, i.e., a battery, connected to one side
of a resonant inductor Lr.
The circuit arrangement shown in FIG. 5 operates as follows. When
switch S1 turns on during a first half-switching cycle (S1 on/S2
off), the input voltage V.sub.in is applied to terminals A and B of
a resonant tank. That is, V.sub.x =V.sub.in. During this first half
switching cycle, inductor Lr stores energy to be released in the
next (i.e., second) half switching cycle (S1 off/S2 on).
During the second half switching cycle (S1 off/S2 on). The voltage
difference between the input voltage, V.sub.in, and capacitor
voltage, V.sub.o, is applied to the terminals A and B of the
resonant tank. It will be shown that the capacitor voltage, equals
nominally twice the input voltage, (2*V.sub.in), during the second
half switching cycle assuming a duty ratio of nominally 0.5 for the
half-bridge switch configuration. In accordance with standard
circuit analysis, it is shown that a voltage (-V.sub.in) is applied
to terminals A and B of the resonant tank during the second half
switching cycle. In sum, the voltage across the resonant tank 50,
i.e., terminals A and B, during the respective half-cycles equals
Vin and -Vin, respectively. This is in contrast to the prior art
circuits of FIG. 4 in which the voltage across the resonant tank 50
is 1/2*V.sub.in to -1/2*V.sub.in, respectively.
FIGS. 6a-6d illustrate typical switching waveforms associated with
the inverter circuit 10 of FIG. 6. Referring first to FIGS. 6a and
6d, as stated above, for a first-half switching cycle (S1 on/S2
off), the voltage across the resonant tank 50, V.sub.x, equals
V.sub.in, (See FIG. 6d).
It is well known in the art that for proper steady state operation,
the average voltage across the terminals A and B of the resonant
tank 50 must be near zero, otherwise the resonant inductor L.sub.r
and transformer T will saturate. Given that the average value of
V.sub.x must be a zero or near zero value, the average value of
V.sub.ds, the body diode voltage of switch S1, must equal the
average value of V.sub.in. During the second half switching cycle
(S1 off/S2 on), V.sub.ds reaches a peak value of 2*V.sub.in, as
shown in FIG. 6c. This peak voltage is realized in part to the
circuit being configured to provide a boost function. Specifically,
a portion of the energy stored in inductor Lr during the first half
switching cycle is released during the second half switching cycle.
This released energy is captured and maintained by clamping
capacitor Co. The voltage on Co is further supplemented by the
input voltage Vin to achieve the peak value 2*V.sub.in during the
second half switching cycle. It is noted that the capacitance value
chosen for clamping capacitor Co is such that the peak voltage is
maintained over multiple cycles.
Given that the average voltage across V.sub.x must be zero or near
zero over a full cycle and recalling that V.sub.x =V.sub.in for the
first half-cycle, V.sub.x must therefore equal (-V.sub.in) the
second half cycle to maintain a zero or near zero value over a full
cycle. During the second half-switching cycle (i.e., S2 on/S1 off)
the circuit voltages of the inverter circuit 10 can be stated
as:
which can be re-written as:
Equation (2) states that the tank excitation voltage, V.sub.x, is
the difference between the input voltage, V.sub.in, and the
clamping capacitor voltage. As described above, during this second
half-cycle the capacitor voltage can be stated as
Substituting Eq. (3) into Eq. (2) yields: ##EQU1##
Voltage V.sub.x for the second half cycle is illustrated in FIG.
6d.
It is appreciated that the average tank excitation voltage of the
inventive circuit is twice that of the prior art circuit of FIG. 4.
As a result, the required turns ratio of the transformer T is
reduced by half. Correspondingly, the leakage inductance is
significantly reduced thereby improving the overall efficiency of
the circuit. In addition, the maximum voltage across the
half-bridge switches is clamped by the capacitor voltage, Vo, and
given as:
where D is the duty ratio of switch S1, which is nominally 0.5. A
further advantage of circuit 10 is that unlike the prior art
circuits where the leakage inductance is dissipated by a snubber
network contributing to circuit inefficiency, the circuit 10 of the
present invention recovers the leakage energy by utilizing a boost
feature.
FIGS. 7-9 illustrate additional embodiments of the inventive
circuit 10 in which the illustrated components have the same
reference symbols as those in FIG. 6.
In FIG. 7, one embodiment of the inventive circuit 10 is shown in
which the resonant inductor L.sub.r is shown in series with the
resonant capacitor C.sub.r while the load is in parallel with the
resonant capacitor.
FIG. 8 shows another embodiment of the inventive circuit 10. In
this embodiment, switch S2 is a P-type MOSFET and further connected
to the negative terminal of clamping capacitor C.sub.0.
FIG. 9 shows another embodiment of the inventive circuit 10. In
this embodiment, the resonant inductor L.sub.r is shown in series
with the resonant capacitor C.sub.r in the load circuit.
In sum, the inventive circuit configuration provides advantages
which are not achievable with the prior art circuit configurations
discussed above. A first advantage realized by the inventive
circuit is a higher efficiency due in part to the leakage
inductance being a part of the resonant inductance. Specifically,
the leakage inductance energy is fully recovered by virtue of being
a part of the resonant inductance thereby precluding the need for a
snubber circuit as used in the prior art. A second associated
advantage is that the voltage across the half-bridge switches is
reduced because of the energy recovery. As a consequence of the low
turns ratio, the associated leakage inductance is minimized. A
third associated advantage is that in addition to the leakage
energy being recoverable it is also reduced as a consequence of the
transformer having a lower turns ratio (i.e., one-half the
conventional turns ratio). The lower turns ratio is achievable
because the inventive circuit tank excitation voltage is twice that
of a conventional excitation voltage.
* * * * *